There are 200-250 interiors that would need the NPCs making controllable. NPCs take up player slots. That means there would need to be an extra 200-250 player slots. That uses a lot of resources.
I have thought about this before, and always come to the idea that I'd probably only do this for certain interiors (Ammunation, Bait Shop, General Store / Gas Station, cheaper clothing shops). I don't think the employee in Didier Sachs would start shooting a robber, but the guy that runs the general store or bait shop might, and the Ammunation guy for sure..
But it's not so simple, I'd have to rework the entire way the holdup system works, to allow some to be actors and some to be NPCs. They are treated very differently.